Method and device for producing three-dimensional objects

ABSTRACT

The invention concerns a method for producing three-dimensional objects ( 3 ) layer by layer using a powdery material ( 5 ) which can be solidified by irradiating it with a high-energy beam.

RELATED APPLICATIONS

This application is a nationalization under 35 U.S.C. 371 ofPCT/SE2008/050562, filed May 14, 2008 and published as WO 2008/147306 A1on Dec. 4, 2008, which claimed priority under 35 U.S.C. 119(e) to U.S.Provisional Patent Application Ser. No. 60/924,440, filed May 15, 2007;which applications and publication are incorporated herein by referenceand made a part hereof.

TECHNICAL FIELD

This invention relates to a method and device for producingthree-dimensional objects layer by layer using a powdery material whichcan be solidified by irradiating it with a high-energy beam.

BACKGROUND ART

Equipment for producing a three-dimensional object layer by layer usinga powdery material which can be solidified, or fused together, byirradiating it with a high-energy beam of electromagnetic radiation orelectrons are known from e.g. U.S. Pat. No. 4,863,538, U.S. Pat. No.5,647,931 and SE524467. Such equipment include for instance a supply ofpowder, means for applying a layer of powder on a vertically adjustableplatform or working area, and means for directing the beam over theworking area. The powder sinters or melts and solidifies as the beammoves over the working area.

When melting or sintering a powder using a high-energy beam, it isimportant to avoid exceeding the vaporization temperature of the powder,since otherwise the powder will merely vaporise instead of forming theintended product. US2005/0186538 discloses a method focusing on thisproblem. In this method a laser beam repeatedly is directed to the samepowder target area during the melting/sintering phase as to stepwiseraise the powder temperature. This way, a too high powder temperature isavoided.

When using an electron beam instead of a laser beam, the situation is insome ways different. As the electron beam hits the powder, a chargedistribution develops around the electron target area. Desirably, thischarge will be led through a produced part of the article to be madeand/or the powder bed towards ground. If the charge distribution densityexceeds a critical limit, an electrical field having a field strengthabove a predetermined level will develop around the position where thebeam is radiating. The electrical field having an electrical fieldstrength above the predetermined level will be referred to as E_(max).An electrical field will cause the powder particles to repel each othersuch that particles leave the uppermost surface layer of the particleand create a distribution of particles floating above the surface. Thefloating particles resemble a cloud positioned above the surface. Whenthe electrical field has a field strength above E_(max), the electricalfield, i.e. the particle cloud, will influence the resolution of thedevice in a negative way. This is partly due to the fact that theparticles in the particle cloud will diverge the electron beam. When theelectrical field has a field strength below E_(max), the electricalfield, i.e. the particle cloud, will not influence the resolution of thedevice in a significant way. A field strength below E_(max) is thusdesirable.

Since the particles are charged they will seek a ground contact andthereby some may leave the cloud and will then contaminate differentparts of the device being positioned inside the vacuum chamber. A resultof such a critical electrical field is that the structure of the powdersurface will be destroyed. Applying the method according toUS2005/0186538 to a powder melting/sintering device equipped with anelectron beam is likely to give a poor result since no measures aretaken in that method to avoid a critical electrical having a fieldstrength above said predetermined level to develop.

One solution to the problem of avoiding discharges is to add conductivematerial, such as carbon, to the powder as to increase the electricalconductivity of the powder. Disadvantages of this solution are, however,that the solidifying process of such a powder mixture may be difficultto control and that the properties of the formed product may be affectedin a negative way. For instance, the mechanical strength may bedecreased.

DISCLOSURE OF INVENTION

The object of this invention is to provide a method and device forlayer-wise production of three-dimensional objects from a powderymaterial, which method and device allow for a controlled and properfusing together of the powdery material, and which are well suited forboth an electron beam. This object is achieved by the method and devicedefined in the independent claims. The dependent claims containadvantageous embodiments, further developments and variants of theinvention.

The invention concerns a method for producing three-dimensional objectslayer by layer using a powdery material, which can be solidified byirradiating it with a high-energy beam. The invention is characterizedin that the method comprises the step of controlling the amount of ionspresent in the close vicinity in the position where the high-energy beamradiates the powdery material. By controlling an amount of ions presentin the close vicinity of the high energy beam it has shown that thetendency to build up a cloud is reduced. Hence the high energy beam,i.e. the electron gun, can operate with a higher output power withoutcreating an electrical field above said predetermined level. This effectis achieved by that the ions in the vicinity of the radiating point ofthe high energy beam reduces the charge density in said vicinity.Naturally, when the high energy beam is an electron gun the ions shouldbe positive. It is advantageous that the number of ions is above a levelrequired to keep the electrical field strength below E_(max). In doingso, enough of the powdery material is neutralised.

In a first embodiment of the invention, the amount of ions present inthe close vicinity in the position where the high-energy beam radiatesis controlled by using the method steps of:

introducing a supplementary gas into the vacuum chamber of theequipment, and

controlling the pressure of the gas to a predefined pressure level,where said supplementary gas is capable of producing ions whenirradiated by the high-energy beam.

It is advantageous that the produced number of ions is above a levelrequired to keep the electrical field strength below E_(max). In doingso, enough of the powdery material is neutralised.

Suitable the vacuum chamber displays a pressure of less than 1*10⁻⁴ mbarbefore the supplementary gas is introduced. The supplementary gas willraise the pressure in the vacuum chamber to be in the interval between1*10⁻¹ mbar and 10⁻⁴ mbar. Preferably the pressure after introduction ofthe supplementary gas is between 10⁻² mbar and 10⁻³ mbar. A higherpressure will lead to more ions being available at the surface of thepowder layer, hence greater power of the electron gun can be acceptedwithout creation of a cloud. On the other hand a high pressure will leadto that the ray from the electron gun will be dispersed to a greaterextent, thereby leading to reduced resolution of the part to beproduced. It has shown that introduction of the supplementary gasbetween 10⁻² mbar and 10⁻³ mbar lead to a good balance between these twoeffects.

In an embodiment of the invention an inert gas is used. Argon isparticularly suitable for use together with production of Titaniumalloys. Helium can also be contemplated. Nitrogen can suitably be usedtogether with Cobalt-Chrome alloys.

In a second embodiment of the invention, the amount of ions present inthe close vicinity in the position where the high-energy beam radiatesis controlled by using the method step of:

introducing ions into the vacuum chamber of the equipment.

By introducing ions into the vacuum chamber, using an ion source such asa sputter device or a plasma device, the number of charged ions in thevacuum chamber can be controlled. Hence, the effect of electron gundispersion can thus be reduced due to the possibility to operate at alow pressure while still having access to a sufficient amount of chargedions. The ion concentration can be obtained by controlling the amount ofions introduced into the vacuum chamber. The ions are preferablydirected in the direction to the powdery material. Depending on thecharge of the particles in the powdery bed, the ions introduced areeither negatively charged or positively charged. The most common case iswhen the powdery bed is negatively charged, thus in this case positivelycharged ions will be used.

Suitably the number of ions introduced into the vacuum chamber iscontrolled in dependence of the output power of the high-energy beam.Since a large portion of the electrons provided from the electron gunwill be discharged via the part of the object made and/or the powder bedto ground, only a small portion of the electrons provided by theelectron gun needs to be compensated for. It is thus preferred to keepthe rate of ions introduced into the vacuum chamber above a levelrequired to keep the electrical field strength below E_(max).

The invention also relates to a device for producing three-dimensionalobjects layer by layer using a powdery material which can be solidifiedby irradiating it with a high-energy beam, which device is adapted to beoperated by at least one of the method steps described above.

BRIEF DESCRIPTION OF DRAWINGS

In the description of the invention given below reference is made to thefollowing figure, in which:

FIG. 1 shows, in a schematic view, an example of a known device forproducing a three-dimensional product, and

FIG. 2 shows, in a schematic view, an example of a device for producinga three-dimensional product in which device a first embodiment of theinventive method can be applied, and

FIG. 3 shows, in a schematic view, an example of a device for producinga three-dimensional product in which device a second embodiment of theinventive method can be applied.

FIG. 4 shows, in a schematic view, an example of the surface of thepowdery material with a charged particle cloud.

EMBODIMENT(S) OF THE INVENTION

FIG. 1 shows an example of a known device 1 for producing athree-dimensional product. The device 1 comprises a verticallyadjustable work table 2 on which a three-dimensional product 3 is to bebuilt up, one or more powder dispensers 4, means 28 arranged todistribute a thin layer of powder on the work table 2 for forming apowder bed 5, a radiation gun 6 in the form of an electron gun fordelivering energy to the powder bed 5 as to fuse together parts of thepowder bed 5, deflection coils 7 for guiding the electron beam emittedby the radiation gun 6 over said work table 2, and a control unit 8arranged to control the various parts of the device 1. In a typical workcycle, the work table 2 is lowered, a new layer of powder is appliedonto the powder bed 5, and the electron beam is scanned over selectedparts of the upper layer 5′ of the powder bed 5. In principal, thiscycle is repeated until the product is finished. An expert in the fieldis familiar with the general function and composition of devices forproducing a three-dimensional product, both with regard to the typeoutlined in FIG. 1. The complete device 1 will constitute a vacuumchamber in which a low pressure is achieved by a vacuum pump (notshown). The pressure in the vacuum chamber is preferably held at apressure level below 10⁻⁴ mbar.

In the case where an electron beam is used, it is necessary to considerthe charge distribution that is created in the powder as the electronshit the powder bed 5. The invention is, at least partly, based on therealization that the charge distribution density depends on thefollowing parameters: beam current, electron velocity (which is given bythe accelerating voltage), beam scanning velocity, powder material andelectrical conductivity of the powder, i.e. mainly the electricalconductivity between the powder grains. The latter is in turn a functionof several parameters, such as temperature, degree of sintering andpowder grain size/size distribution.

Thus, for a given powder, i.e. a powder of a certain material with acertain grain size distribution, and a given accelerating voltage, it ispossible, by varying the beam current (and thus the beam power) and thebeam scanning velocity, to affect the charge distribution.

By varying these parameters in a controlled way, the electricalconductivity of the powder can gradually be increased by increasing thetemperature of the powder. A powder that has a high temperature obtainsa considerably higher conductivity which results in a lower density ofthe charge distribution since the charges quickly can diffuse over alarge region. This effect is enhanced if the powder is allowed to beslightly sintered during the pre-heating process. When the conductivityhas become sufficiently high, the powder can be fused together, i.e.melted or fully sintered, with arbitrary values of the beam current andbeam scanning velocity.

A general function for describing the charge density that develops inthe powder in an arbitrary scanning procedure will be a rather complexfunction of time and beam position since the charge density generatedalong one scanned path will be affected by the charge density generatedalong another scanned path if these paths are not very well separated inspace and time. Thus, summation effects between different paths must betaken into account.

FIG. 2 shows an example of a device 21 for producing a three-dimensionalproduct in which a first embodiment of the inventive method can be used.The device 21 comprises, apart from the parts described in FIG. 1, alsoa gas inlet 22 in to which a supplementary gas can be introduced intothe vacuum chamber of the device. The gas inlet is regulated by acontrollable valve 23 that can be controlled by the control unit 8. Thecontrollable valve 23, and thus the gas pressure in the vacuum chamber,may be controlled with respect to the pressure in the vacuum chamber.This pressure can be measured with a pressure sensor 24 connected to thecontrol unit 8.

FIG. 3 shows an example of a device 31 for producing a three-dimensionalproduct in which a second embodiment of the inventive method can beused. The device 31 comprises, apart from the parts described in FIG. 1,also a sputter device 32 by which ions can be introduced into the vacuumchamber of the device. The sputter device 32 may be controlled by thecontrol unit 8. The gas used in the sputter device 32 to create the ionsis preferably an inert gas such as Argon. The pressure in the vacuumchamber is kept at a predefined level, preferably below 10⁻⁴ mbar. Thepressure in the vacuum chamber can be measured with a pressure sensor 34connected to the control unit 8.

FIG. 4 shows the upper layer 5′ of the powder bed 5 of the powderymaterial with a charged particle cloud 41. The cloud is concentratedaround the position where the electron beam 42 radiates the powderymaterial. With a higher electrical field, a larger cloud will occuraround the radiating point. It is therefore an object of the inventionto limit the size, i.e. height, of the cloud. The number of ionsintroduced into the vacuum chamber or created in the vacuum chambershould thus be above a predefined level in order to neutralise enoughcharges in the cloud. The predefined level should be selected such thatit keeps the electrical field strength below E_(max). In doing so,enough of the powdery material is neutralised.

The invention is not to be regarded as being limited to the embodimentsdescribed above, a number of additional variants and modifications beingpossible within the scope of the subsequent patent claims.

What is claimed is:
 1. A method of producing three-dimensional objects, layer by layer, by solidifying a powdery material, comprising the steps of: solidifying a powdery material within a vacuum chamber by irradiating the powdery material with a high-energy beam and generating charged particles of the powder, introducing a supplementary inert gas into the vacuum chamber, and controlling the pressure of the inert gas to a predefined pressure level, and producing a predefined amount of ions of the inert gas when the powder is irradiated by the high-energy beam.
 2. The method according to claim 1, wherein the predefined number of ions is above a level required to keep the electrical field strength below E_(max).
 3. The method according to claim 1, wherein the vacuum chamber has a pressure of less than 1*10⁻⁴ mbar before the supplementary gas is introduced.
 4. The method according to claim 1, wherein the pressure in the vacuum chamber has a pressure in the range between 1*10⁻¹ mbar and 1*10⁻⁴ mbar after the supplementary gas is introduced.
 5. The method according to claim 1, wherein the inert gas is at least argon, helium or nitrogen gas.
 6. The method according to claim 5, wherein the inert gas is argon.
 7. The method according to claim 5, wherein the inert gas is helium.
 8. The method according to claim 1, wherein the pressure in the vacuum chamber is controlled in dependence of the output power of the high-energy beam to substantially balance the amount of ions of inert gas produced with the amount of charged particles produced.
 9. A method for producing three-dimensional objects, layer by layer, by solidifying a powdery material by irradiating the powdery material with an electron beam, comprising the steps of: providing a radiation gun that emits an electron beam, introducing a supplementary inert gas and the powdery material into a vacuum chamber, solidifying the powdery material within the vacuum chamber with the electron beam and generating charged particles of the powder, controlling the pressure of the inert gas to a predefined pressure level, and producing a predefined amount of ions of the inert gas by irradiating the gas with the electron beam.
 10. The method according to claim 5, wherein the inert gas is nitrogen. 